Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), phase change random access memory (PCRAM), and flash memory, among other types of memory.
Resistance variable memory devices, such as PCRAM devices, can include a structural phase change material such as a chalcogenide alloy, for instance, which can be programmed into different resistivity states to store data. The phase change memory cells are nonvolatile and the particular data stored in a phase change memory cell can be read by sensing the cell's resistance, e.g., by sensing current and/or voltage variations based on the resistance of the phase change material.
In cases in which the resistance variable memory device includes a chalcogenide alloy, the chalcogenide alloy can exhibit a reversible structural phase change, e.g., from amorphous to crystalline. A small volume of the chalcogenide alloy can be integrated into a circuit that can allow the cell to act as a fast switching programmable resistor. This programmable resistor can exhibit a greater dynamic range of resistivity between the crystalline state (low resistivity) and the amorphous state (high resistivity), and is also capable of exhibiting multiple intermediate states that allow multi-bit storage in each cell. That is, resistance variable memories may achieve multi-level cell (MLC) functionality via programming of memory cells to one of a number of different resistance levels.
The reliability of phase change memory cells over time can be affected by various factors such as the isolation between the phase change memory cell in an array and underlying substrate and/or the current densities used to provide sufficient power to alter the phase change material, among other factors. As an example, parasitic resistance can be introduced to a cell from adjacent cells that are not properly isolated. This parasitic resistance can cause undesired electro-migration in conductive materials of a phase change memory cell and may cause phase separation in the phase change material, in some instances.